The long term objective of these studies is to 1) identify, 2) characterize and 3) examine the regulation of genes important in normal mammalian development. Our approach involves using murine retroviruses to induce mutations in mice that affect the developmental process. Mutations induced in this manner are particularly amenable to experimental intervention since the viral sequences serve as useful markers to molecularly clone and characterize both mutant and wild-type alleles. Mutations can be induced by microinjecting DNA into mouse zygotes or by infecting embryos with virus. However, this has proven to be a difficult and laborious process. A simpler and more direct approach involves using highly viremic mouse strains which spontaneously acquire new ecotropic viral sequence within their germline, presumably due to germline infection by the virus they spontaneously produce. However, this is a low frequency event and, for this approach to be feasible, it requires the identification of mice that amplify their endogenous proviral content at high frequencies. Recently, we have found such mice. Unlike RF/J mice whose endogenous ecotropic proviral content is stable, the endogenous proviral content of RF/J-derived hybrid mice (RF/J mice backcrossed to SWR/J mice) is not stable. Both amplification and deletion of ecotropic proviral DNA sequences occurs. As many as 50% of the progeny is some litters contained new ecotropic proviral sequences and from one to six new proviruses were acquired by an individual animal. These sequences were acquired early in development, at different developmental stages, and were always present in the germline. In all, more than 40 newly acquired ecotropic proviral loci were detected in only 120 mice analyzed. Therefore, under certain circumstances, high frequency germline virus amplification occurs. It is the goal of these studies to (1) identify the mechanism(s) of virus instability in RF/J-derived hybrid mice, (2) determine if any of the newly acquired proviruses identified in RF/J-derived hybrid mice are insertional mutagens, (3) measure the frequency of virus amplification in other types of hybrid mice and identify factors that increase the amplification frequency, and finally (4) use these mice and their associated retroviruses to generate and characterize mutations that affect the developmental process.